Decadal Shift in El Niño Influences on Indo–Western Pacific and East Asian Climate in the 1970s*

Abstract

El Niñ o's influence on the subtropical northwest (NW) Pacific climate increased after the climate regime shift of the 1970s. This is manifested in well-organized atmospheric anomalies of suppressed convection and a surface anticyclone during the summer (June-August) of the El Niñ o decay year [JJA(1)], a season when equatorial Pacific sea surface temperature (SST) anomalies have dissipated. In situ observations and oceanatmospheric reanalyses are used to investigate mechanisms for the interdecadal change. During JJA(1), the influence of the El Niñ o-Southern Oscillation (ENSO) on the NW Pacific is indirect, being mediated by SST conditions over the tropical Indian Ocean (TIO). The results here show that interdecadal change in this influence is due to changes in the TIO response to ENSO.During the postregime shift epoch, the El Niñ o teleconnection excites downwelling Rossby waves in the south TIO by anticyclonic wind curls. These Rossby waves propagate slowly westward, causing persistent SST warming over the thermocline ridge in the southwest TIO. The ocean warming induces an antisymmetric wind pattern across the equator, and the anomalous northeasterlies cause the north Indian Ocean to warm through JJA(1) by reducing the southwesterly monsoon winds. The TIO warming excites a warm Kelvin wave in tropospheric temperature, resulting in robust atmospheric anomalies over the NW Pacific that include the surface anticyclone. During the preregime shift epoch, ENSO is significantly weaker in variance and decays earlier than during the recent epoch. Compared to the epoch after the mid-1970s, SST and wind anomalies over the TIO are similar during the developing and mature phases of ENSO but are very weak during the decay phase. Specifically, the southern TIO Rossby waves are weaker, so are the antisymmetric wind pattern and the North Indian Ocean warming during JJA(1). Without the anchor in the TIO warming, atmospheric anomalies over the NW Pacific fail to develop during JJA(1) prior to the mid-1970s. The relationship of the interdecadal change to global warming and implications for the East Asian summer monsoon are discussed.

Relative role of El Niño and IOD forcing on the southern tropical Indian Ocean Rossby waves

“…IOD events have cooccurred with El Niño more frequently after the mid-1970s climate shift [e.g., Chakravorty et al, 2014]. Recent studies have revealed that after the mid-1970s climate shift, the ocean dynamics have acted as a major driver in modulating the sea surface temperature (SST) at interannual time scale mainly over the equatorial eastern Indian Ocean (EIO) and southern TIO (STIO) [Xie et al, 2010;Chakravorty et al, 2014]. In particular, SST variability in the southwestern TIO is closely associated with the westward propagating Rossby waves partly generated by the El Niño-induced anticyclonic wind stress curl in the eastern basin Kinter, 2001, 2002;Xie et al, 2002;Chowdary et al, 2009;Schott et al, 2009;Gnanaseelan and Vaid, 2010].…”

Attribution of the summer warming since 1970s in Indian Ocean Basin to the inter‐decadal change in the seasonal timing of El Niño decay phase

“…The long-term IOB SST warming has commonly been attributed to the anthropogenic effect associated with global warming [Barnett et al, 2005;Pierce et al, 2006]. Nevertheless, decadal changes over the Indian Ocean have been found closely related to ENSO [Cole et al, 2000;Allan et al, 1995;Reason et al, 2000;Xie et al, 2010;Chowdary et al, 2012]. For example, Allan et al [1995] suggested that the epochal changes in the mean condition over the south Indian Ocean are in-phase with ENSO-related correlation patterns.…”

Impacts of tropical Indian Ocean SST on the meridional displacement of East Asian jet in boreal summer

“…Owing to the thermocline shoaling in the southwest TIO and the strengthening of ENSO intensity, interannual variability of TIO SST is intensified after the late 1970s Xie et al, 2010). During this epoch, the TIO warms in response to El Niño, and this warming then influences the climate in the Indian Ocean (IO) and surrounding regions after El Niño decays (Yang et al, 2007;Xie et al, 2009).…”